The communication scheme subsequent to W-CDMA (Wideband Code Division Multiple Access) and HSDPA (High Speed Downlink Packet Access), that is, long-term evolution (LTE), has been set forth by 3GPP, which is the standards organization of W-CDMA, and, for radio access schemes, OFDMA (Orthogonal Frequency Division Multiple Access) has been employed on the downlink and SC-FDMA (Single-Carrier Frequency Division Multiple Access) has been employed on the uplink.
OFDMA is a scheme to perform transmission by dividing a frequency band into a plurality of narrow frequency bands (subcarriers) and placing data on each frequency band, and, by arranging subcarriers on frequencies densely so as to partly overlap each other and yet not interfere with each other, it is possible to realize high-speed transmission and improve the efficiency of use of frequencies.
SC-FDMA is a transmission scheme that can reduce interference between terminals by dividing a frequency band and performing transmission using different frequency bands between a plurality of terminals. SC-FDMA has a characteristic of reducing the variation of transmission power, so that low power consumption of terminals and wide coverage are made possible.
LTE is a system in which communication is performed by sharing one or two or more physical channels between a plurality of mobile stations (UE: User Equipment) on both the uplink and the downlink. The above channels shared by a plurality of mobile stations UE are generally referred to as “shared channels,” and, in LTE, these include the PUSCH (Physical Uplink Shared Channel) for the uplink and the PDSCH (Physical Downlink Shared Channel) for the downlink.
Then, in a communication system using the above-described shared channels, it is necessary to signal, per subframe which is a transmission time unit, to which mobile stations UE the above shared channels are assigned. A subframe may be referred to as a “transmission time interval” (TTI).
In LTE, the PDCCH (Physical Downlink Control Channel) is set forth as a downlink control channel to be used for the above signaling, and, furthermore, the PCFICH (Physical Control Format Indicator Channel) is set forth as a control channel to report the number of OFDM symbols used for the PDCCH, and the PHICH (Physical Hybrid-ARQ Indicator Channel) is set forth as a control channel to transmit hybrid ARQ ACK or NACK information for the PUSCH.
Downlink control information that is transmitted by the PDCCH includes, for example, downlink scheduling information, UL scheduling grant, overload indicator and transmission power control command bit (non-patent literature 1). Also, the above downlink scheduling information includes, for example, downlink resource block assignment information, UE IDs, the number of streams, information related to precoding vectors, data size, modulation scheme, and information related to HARQ (Hybrid Automatic Repeat reQuest). Furthermore, the above uplink scheduling grant includes, for example, uplink resource block assignment information, UE IDs, data size, modulation scheme, uplink transmission power information, and demodulation reference signal information.
The above PCFICH is information to report the PDCCH format. To be more specific, by means of this PCFICH, the number of OFDM symbols to which the PDCCH is mapped, is reported as control channel format information (CFI). In LTE, the number of OFDM symbols to which the PDCCH is mapped is one of 1, 2 and 3, and, in one subframe, the PDCCH is mapped from the top OFDM symbol (non-patent literature 2).
On the downlink, a range corresponding to the number of OFDM symbols (CFI value) reported by the PCFICH from the beginning of a subframe, serves as a control channel region assigned to the PDCCH. A mobile station decodes the control channel region, and, if there is information addressed to that mobile station, further specifies and decodes the radio resources allocated to the PDSCH, based on downlink control information.